The invention relates to a servo control system, especially a power steering system for motor vehicles of the type which include:
a shaft which serves as an actuating member or is coupled thereto and which is subdivided axially into an input-side primary-shaft part and an output-side secondary-shaft part, PA1 a spring arrangement which drive-connects the shaft parts and which allows a relative rotation between the shaft parts, PA1 a servo-valve arrangement working in dependence on the relative rotation, with an inflow connection assigned to a pressure source or pump, with an outflow connection assigned to an essentially pressureless reservoir, and with two working connections which are assigned to a servomotor and between which a pressure difference dependent on the amount and direction of the relative rotation can be generated, and PA1 a fluidic reaction control for generating a reaction force counteracting the particular direction of actuation of the shaft, the one shaft part being arranged, for the purpose of forming the reaction control, with a portion within a bush part which is arranged fixedly in terms of rotation on the other shaft part and which, together with the portion, forms a double-acting rotary-blade unit with oppositely acting chambers, in which the pressure in one chamber seeks to bring about a relative rotation of the bush part and portion in one direction and the pressure in the other chamber seeks to bring about a relative rotation in the other direction.
Servo controls of this type are known, for example, from German patent document DE-A 2,412,548. Where a vehicle steering is concerned, as result of the reaction control, some resistance can be felt on the steering handwheel, at least within a middle range of the possible steering angles, and this resistance is correlated with the steering forces actually to be exerted. The driver is thereby given a good feel for the particular driving state.
In this respect, it is basically known to cause the reaction control to work in a parameter-dependent manner, for example in dependence on the driving speed, for instance in such a way that the steering becomes heavier at a higher driving speed.
Reference can be made, in this regard, to German patent document DE-C-3,690,102 from which it is known to connect the inflow of a fluidic reaction control to the pressure source via a throttle valve controllable in a parameter-dependent manner.
An object of the invention, in a servo control of the type mentioned in the introduction, is to produce an especially simple construction.
This object is achieved, according to the invention, in that the rotary-blade unit has two chambers, in that each chamber can be connected to the reservoir via a throttle and to an inflow via a controllable slot between mutually cooperating control edges on the bush part and on the portion, in that the two slots are both at least virtually closed in a middle position of the bush part and portion, whilst a respective slot opens in the event of a relative rotation between the bush part and portion, and in that the inflow is connected to the pressure source via a throttle valve controllable in a parameter-dependent manner.
In the invention, therefore, the reaction control constitutes a member which can, in principle, be separated from the servo-valve arrangement and which is fluidically connected in parallel with the servo-valve arrangement between the pump or pressure source and the reservoir. This affords the advantageous possibility of allowing a modular construction, in order to produce and arrange the servo control or power steering with or without a reaction control, as required.
It is advantageous, furthermore, if the throttle valve is arranged in series in front of the rotary-blade or rotary-piston unit. This affords the advantage, in comparison with a basically possible arrangement of the controllable throttle valve between the rotary-blade or rotary-piston unit and the reservoir, that the chambers of the rotary-blade or rotary-piston unit are loaded only by comparatively moderate pressures and the respective sealing elements are correspondingly subjected to comparatively little load. Moreover, in the middle position, the chambers of the rotary-blade or rotary-piston unit are substantially pressure-free because the slots are then (virtually) closed, with the result that the seals of the chambers are subjected to hardly any stress in the very frequently occurring middle position.
This affords the further advantage that the seals can work with a low pressing force and accordingly with low friction, with the result that a good operating behavior of the power steering, with low hysteresis, becomes possible.
According to an especially preferred embodiment of the invention, the shaft parts are connected to one another via a torsion bar which passes through an axial cavity in the one shaft part so as to leave an annular space free, and this annular space is connected to the reservoir and, via throttle bores passing through the wall of the shaft part, is connected to the chambers of the rotary-blade or rotary-piston unit. An especially compact and slim design can be achieved thereby.
A so-called cut-off valve is expediently arranged between the inflow and throttle valve and, above a threshold value of the pressure of the inflow, breaks its connection to the controllable throttle valve and therefore to the pump or pressure source. In this way, the reaction force occurring can be limited to a desired value in a constructively simple way.
The controllable throttle valve is typically controlled in a parameter-dependent manner by means of an electronic control circuit. Provision is made here, in the conventional way, for the control circuit constantly to check itself for malfunctions and to stop if faults occur. In this case, the actuating member of the controllable throttle valve, usually an electromagnet, is isolated from its energy supply. At the same time, the controllable throttle valve is expediently designed so that it then assumes a relatively unthrottled state, in which the reaction forces which can be generated are comparatively high. Where a power steering of a motor vehicle is concerned, this guarantees that the steering works in the operating mode for a high driving speed and a high degree of safety accordingly remains guaranteed. It is merely necessary to allow for some loss of comfort, since the steering then works somewhat more heavily than usual at low driving speeds.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.